Minimally Invasive Glaucoma Surgeries
Minimally invasive glaucoma surgery (MIGS), also called microinvasive or microincisional glaucoma surgery, refers to procedures that enhance preexisting pathways for aqueous outflow. Approaches include decreasing trabecular meshwork resistance to aqueous flow, improving aqueous flow through Schlemm canal, and/or creating a low-resistance pathway for aqueous flow between the anterior chamber and the suprachoroidal space. Despite the name, MIGS procedures are incisional surgeries, are invasive, and carry risk. However, significant complications occur less frequently with MIGS than with traditional glaucoma surgeries.
Because MIGS aims to increase flow through preexisting pathways, the efficacy of most of the procedures (other than those that shunt aqueous to the suprachoroidal space) is limited by the episcleral venous pressure (EVP) and distal outflow resistance. In most patients, the EVP is 6–9 mm Hg. Typically, MIGS procedures lead to IOPs in the mid to high teens.
MIGS procedures fall into 3 general categories:
Indications and contraindications
Angle-based MIGS necessitates a gonioscopic view of the anterior chamber angle during surgery. Developing appropriate eye-hand coordination with a blunt-tipped instrument (such as a cyclodialysis spatula) while visualizing the angle with a gonioprism is useful in attaining the dexterity required to perform angle-based MIGS.
Postoperative management involves the use of steroids and antibiotics. For procedures that require TM disruption, pilocarpine is also used. Complications include hyphema, postoperative IOP spikes, prolonged postoperative inflammation, obstruction of the device by iris tissue, improper insertion of the device, cyclodialysis, iridodialysis, corneal decompensation, and hypotony.
There is limited evidence to guide the use of these devices in advanced glaucoma, and they are not well suited for eyes that require a very low target IOP. MIGS procedures are often performed in conjunction with cataract surgery. Indications for MIGS include
Trabecular bypass devices
Trabecular bypass devices are designed to create a low-resistance pathway between the anterior chamber and Schlemm canal.
iStent The first-generation iStent (Glaukos Corporation) has an L-shaped design (Fig 13-19A). The sharp tip of the iStent is designed to pierce the TM tangentially. There are several retention rings on the outer aspect of the hollow body of the iStent, which are designed to keep the stent in place.
The second-generation iStent (iStent inject) has a different design, with a “head” located in Schlemm canal, a “thorax” that is lodged in the trabecular meshwork, and a flange that remains in the anterior chamber (Fig 13-19B). Two iStent inject devices are implanted in the same eye.
An incision is made in the temporal cornea, and the eye is filled with a cohesive viscoelastic. The patient’s head is tilted away from the surgeon, and the microscope is adjusted appropriately. The angle is visualized with a direct gonioscope.
With the first-generation iStent, the device tangentially approaches the trabecular meshwork, and the tip of the iStent pierces the TM and is advanced into Schlemm canal. When the device is in appropriate position, it is released from the handpiece with the push of a button (Video 13-14).
Courtesy of Shakeel Shareef, MD.
With the second-generation device (iStent inject), the preloaded handpiece is introduced into the eye and the protective sleeve is retracted, exposing the delivery device tip. The tip is embedded into the TM and a button is pushed to deploy the stent. A second preloaded stent is inserted 2 to 3 clock-hours away (Video 13-15).
Figure 13-19 Illustration of the iStent. A, In the first-generation iStent, the tip pierces the trabecular meshwork (TM), allowing the device to slide into the Schlemm canal. Retention rings hold the device in place. The snorkel is open to the anterior chamber, allowing aqueous to flow through the device into Schlemm canal. Arrows indicate aqueous flow. B, The head of the iStent inject contains 4 equal-size and evenly spaced ports for fluid passage (arrow). The head is connected to a narrow thorax that attaches to a wider flange region. An inlet port runs the entire length of the iStent inject.
(Part A illustration courtesy of Mark Miller; part B courtesy of Bahler CK, Hann CR, Fijeld T, Haffner D, Heitzmann H, Fautsch MP. Second-generation trabecular meshwork bypass stent (iStent inject) increases outflow facility in cultured human anterior segments. Am J Ophthalmol. 2012;153(6):1206–1213.)
Courtesy of Wayne Tie, MD.
A 2-year study comparing cataract surgery combined with implantation of the first-generation iStent and cataract surgery alone found that patients receiving the iStent were more likely to achieve a 20% reduction in unmedicated IOP (53% vs 44%), though this was not statistically significant. iStent patients were statistically more likely to have an unmedicated IOP ≤21 mm Hg (61% vs 50%).
In a similar 2-year study of the iStent inject device, patients underwent a medication washout at baseline and at the conclusion of the study. In the microstent group, mean diurnal IOP reduction was greater (7.0 ± 4.0 mm Hg vs 5.4 ± 3.7 mm Hg; P < .01), and there was a higher likelihood of achieving a 20% reduction in unmedicated IOP (76% vs 62%).
Craven ER, Katz LJ, Wells JM, Giamporcaro JE; iStent Study Group. Cataract surgery with trabecular micro-bypass stent implantation in patients with mild-to-moderate open-angle glaucoma and cataract: two-year follow up. J Cataract Refract Surg. 2012:38(8):1339–1345.
Hydrus The Hydrus implant (Ivantis) is similar to the iStent in that the Hydrus is designed to traverse the TM. However, this device is also designed to dilate the Schlemm canal over approximately 3 clock-hours.
The initial steps are similar to those for iStent. The inserter containing the Hydrus stent is introduced into the anterior chamber, and the tip of the cannula perforates the trabecular meshwork, allowing for delivery of the Hydrus into Schlemm canal (Video 13-16).
Courtesy of Iqbal “Ike” Ahmed, MD.
A 2-year study comparing cataract surgery combined with implantation of the Hydrus device versus cataract surgery alone found that patients who received the Hydrus were more likely to achieve a statistically significant 20% reduction in unmedicated IOP (80% vs 46%). Mean diurnal IOP reduction was greater in the Hydrus group as well (9.4 mm Hg vs 7.4 mm Hg).
Pfeiffer N, Garcia-Feijoo J, Martinez-de-la-Casa JM, et al. A randomized trial of a Schlemm’s canal microstent with phacoemulsification for reducing intraocular pressure in open-angle glaucoma. Ophthalmology. 2015;122(7):1283–1293.
These procedures are designed to disrupt or remove the inner wall of Schlemm canal and TM, opening the canal and downstream collector channels directly to aqueous outflow. In addition to topical steroids and antibiotics, pilocarpine is used to prevent the development of PAS.
The Trabectome (MicroSurgical Technologies) is used to ablate the TM and inner wall of Schlemm canal. The device consists of a handpiece with a tip that ablates the tissue by means of a bipolar 550 kHz electrode. There are also irrigating and aspirating ports to dissipate heat, remove debris liberated during the procedure, and maintain anterior chamber stability.
A temporal clear corneal incision is created, and a gonioscopy lens is used (Video 13-17). Unlike other angle-based procedures, viscoelastic is not employed during the surgery because of concerns that gas bubbles resulting from ablation could be trapped in the viscoelastic and obscure the view of the trabecular meshwork. The TM and inner wall of the Schlemm canal are removed by the ablating tip of the handpiece. Up to 180° of TM is excised during the procedure. Postoperatively, viscoelastic may be placed in the eye to prevent blood reflux from the exposed collector channels.
Trabectome: setup and procedure.
Courtesy of Sameh Mosaed, MD.
No prospective randomized trials have yet compared the efficacy of the Trabectome to that of cataract surgery alone.
Kahook Dual Blade
Conventional goniotomy (see Chapter 11) is most effective in primary congenital glaucoma; however, it has been attempted in other forms of glaucoma as well. The Kahook Dual Blade (New World Medical) is a device designed to allow the surgeon to perform an excisional goniotomy by removing of a strip of tissue from the TM and inner wall of Schlemm canal (Video 13-18).
Kahook Dual Blade goniotomy.
Courtesy of Iqbal “Ike” Ahmed, MD.
An incision is created in the temporal cornea, and the eye is filled with a cohesive viscoelastic. The nasal angle is visualized using a direct goniolens. The device tip is embedded into the trabecular meshwork, and the TM and inner wall of Schlemm canal are stripped and excised.
No prospective randomized controlled studies have evaluated goniotomy with the Kahook Dual Blade.
Sieck EG, Epstein RS, Kennedy JB, et al. Outcomes of Kahook Dual Blade goniotomy with and without phacoemulsification cataract extraction. Ophthalmol Glaucoma. 2018;1(1):75–81.
Gonioscopy-assisted transluminal trabeculotomy
Gonioscopy-assisted transluminal trabeculotomy (GATT) is similar to ab externo 360° suture trabeculotomy (see Chapter 11), except that the suture is introduced into Schlemm canal through an intracameral approach (Videos 13-19, 13-20). An alternate term for GATT is 360° ab interno suture trabeculotomy. An advantage of GATT over the ab externo procedure is that it spares the conjunctiva.
GATT and ab interno canaloplasty.
Courtesy of Iqbal “Ike” Ahmed, MD.
Ab externo trabeculotomy.
Courtesy of Lauren Blieden, MD.
Two paracenteses are created: 1 for intracameral surgical manipulations and the other for entry of a suture or illuminated microcatheter. The eye is filled with a cohesive viscoelastic. A goniotomy blade is used to create a 1–2-mm goniotomy in the nasal angle. A suture or microcatheter is introduced into the Schlemm canal through the goniotomy site. The device is passed 360°, and the distal and proximal ends of the suture are grasped and used to cheese-wire the TM/Schlemm canal complex, creating a 360° trabeculotomy. The chamber is partially filled with viscoelastic to mitigate postoperative bleeding.
No prospective comparative studies have been performed. A retrospective study of 198 eyes showed an average reduction in IOP of 9 mm Hg in POAG patients and of 14 mm Hg in secondary open-angle glaucoma patients at 24 months after GATT. In addition, patients were on 1–2 fewer medications at 24 months.
Grover DS, Smith O, Fellman RL, et al. Gonioscopy-assisted transluminal trabeculotomy: an ab interno circumferential trabeculotomy: 24 months follow-up. J Glaucoma. 2018;27(5):393–401.
Ab interno canaloplasty
Ab interno canaloplasty (AbIC) uses a specially designed illuminated microcatheter that is introduced into Schlemm canal via an internal approach. Viscoelastic is injected through the microcatheter into Schlemm canal to viscodilate the canal and, possibly, the downstream collector channels.
The procedure is similar to that of GATT; however, viscoelastic is injected while the catheter is advanced into the canal, and a goniotomy is not performed unless specifically desired (see Video 13-19).
No prospective trials have evaluated AbIC.
Aqueous shunt into the suprachoroidal space
Currently, there are no devices approved by the US Food and Drug Administration (FDA) to shunt aqueous into the suprachoroidal space. The CyPass Micro-Stent (Alcon), a tube-shaped device placed in the anterior chamber angle to create a conduit from the anterior chamber to the suprachoroidal space, was formerly available. In a 2-year prospective randomized controlled trial, CyPass combined with cataract surgery demonstrated an additional IOP reduction of 2.0 mm Hg below baseline compared with cataract surgery alone. Despite its efficacy, the device was recalled by the FDA in 2018 because increased endothelial cell loss was observed during extended observation of patients in the pivotal clinical trial.
Vold S, Ahmed II, Craven ER, et al; CyPass Study Group. Two-year COMPASS trial results: supraciliary microstenting with phacoemulsification in patients with open-angle glaucoma and cataracts. Ophthalmology. 2016;123(10):2103–2112.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.